1. Field of the Invention
The present invention relates to a method of preparing a ceramic ingot of a spent filter by grinding and sintering a spent filter having trapped radioactive cesium.
2. Discussion of Related Art
In recent years, it has been reported that electric power currently produced by nuclear power generation accounts for approximately 40% of electric power production in Korea, and spent nuclear fuel formed from the nuclear power generation amounts to 850 tons every year. Pyroprocessing is emerging as an option for recycling such spent nuclear fuel. Pyroprocessing is technology of recovering nuclear fuel materials such as uranium (U) and transuranium (TRU) elements from spent nuclear fuel using an electrochemical method in a molten salt medium having a high temperature (500 to 650° C.). A high-temperature voloxidation process is accompanied with a pretreatment process of the pyroprocessing. In this case, a volatile nuclide (Kr, Xe, C-14, H-3, etc.) and a semi-volatile nuclide (Cs, Tc, I, etc.) are released. As a result, a demand for technology of treating a filter trapping the nuclides to prevent release of the nuclides to the facilities and environments and technology of preparing the nuclides into a stable solid-phase ingot so as to dispose of the filter has been emphasized.
In this regard, Korean Patent Nos. 0192128 and 1090344 disclose a technique for selectively separating and trapping cesium through a high-temperature chemical adsorption method using a solid medium including fly ash and silica at 40 to 65% by weight, alumina at 15 to 30% by weight, an iron oxide at 5 to 15% by weight, a molybdenum oxide at 1 to 15% by weight, a chromium oxide at 1 to 10% by weight, and a vanadium oxide at 1 to 10% by weight. In the prior-art patents, it was proposed that radiation and heat of the spent nuclear fuel may be reduced by selectively trapping cesium (Cs-137), which is a highly radioactive and high heat generation nuclide, and has a half-life of approximately 30 years, in stable phases of cesium aluminosilicate (pollucite (CsAlSi2O6), CsAlSiO4, and CsAlSi5O12) using a ceramic filter made of an aluminosilicate component, thereby reducing an area required for disposal. Here, at least 99% of the cesium (Cs-137) is expected to volatilize during a high-temperature heat treatment process of pyroprocessing pretreatment.
Also, a vitrification method has often been used to dispose of a high level of radioactive waste. In this method, radioactive nuclides are trapped in a physicochemically hard glass structure, and solidified into a glass-solidified mass. Although the glass-solidified mass prepared thus has advantages in that it is adaptable to compositions and process parameters, and highly resistant to radioactivity, it has a problem of low leach resistance. According to the literature by W. J. Gray, 5.5%, 31% and 100% of cesium volatilizes at 1,000° C., 1,100° C. and 1,200° C., respectively, during this process. Therefore, there is a difficulty in a process of retrapping volatilized cesium (W. J., “Volatility of Some Potential High-Level Radioactive Waste Forms,” Rad. Waste Mgmt., 1(2), 147-169 (1980)), and cesium may be included in a glass-solidified mass at a content of up to 15% by weight, and an increase in an added solidifying agent may result in an increase in an amount of waste.
In addition, when a glass-solidified mass is used in a gamma-ray irradiator, it is not effectively used in the field of industry to apply gamma rays due to low specific radioactivity (Ci/g) (International Atomic Energy Agency, Feasibility of Separation and Utilization of Cesium and Strontium from High Level Liquid Waste, Technical Report Series No. 356, IAEA, Vienna (1993)).
As described above, technology of trapping volatile cesium and technology of vitrifying a high level of radioactive waste are known in the related art, but have various problems. Therefore, there is a demand for a new method of preparing a filter into a solidified mass, wherein the filter serves to trap gas-phase radioactive waste formed from spent nuclear fuel during a high-temperature heat treatment process.